Spinal fixation systems such as pedicle screw and rod constructs are commonly used to promote fusion between intervertebral bodies. The insertion of pedicle screws typically requires a linear “line-of-approach” trajectory that is aligned with the longitudinal axis of the screw, in order to accommodate the access and delivery instruments. Similarly, anchors such as bone screws may be used to directly fix intervertebral implants to vertebral bodies, typically requiring the insertion of several screws at unique angles oblique to the sagittal and/or transverse plane, and thus multiple lines-of-approach. However, in a variety of surgical situations, achieving a desired trajectory for screw insertion can be difficult due to the patient's anatomy obstructing a linear line-of-approach. For example, medially-directed placement of pedicle screws into the sacrum is desirable to prevent screw loosening and/or pullout, but can be prohibited due to the iliac crest obstructing the linear line-of-approach.
In addition to the above-discussed linear line-of-approach problems, limitations of the fixation anchors themselves can further limit spinal fixation treatment approaches. For example, unilateral spinal fixation procedures, wherein a pedicle screw and rod construct is placed on a single side of the spine, provide advantages such as limiting surgical site morbidity and shortening surgical time when compared with standard bilateral fixation procedures wherein the construct is placed on both sides of the spine and interconnected. However, unilateral fusion constructs typically exhibit decreased mechanical rigidity in comparison to bilateral constructs, for example due to lower torsional and/or rotational rigidity and weaker resistance to screw pullout forces under physiologic loading when compared to typical bilateral constructs. As a result, unilateral fixation procedures are rarely performed in lieu of bilateral fixation procedures.
What is therefore desirable are spinal fixation systems that allow for the creation of rigid constructs when the linear line-of-approach for insertion of fixation anchors is unavailable and/or undesirable (e.g., when multiple anchors are required), while at the same time providing increased rigidity and robustness to spinal constructs such as those used in unilateral fusion procedures.